OPA320 datasheet(17/37 Pages) NSC

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100kW

IN+

OPA320

V+

V-

OUT

OPA320

OUT

V+

10MW

<1pF

(1)

BIAS

OPA320, OPA2320

OPA320S, OPA2320S

www.ti.com

SBOS513E – AUGUST 2010 – REVISED JUNE 2013

For single-supply applications, the +IN input can be

photodiode

can

significantly

reduce

its

biased with a positive dc voltage to allow the output

capacitance. Smaller photodiodes have lower

to reach true zero when the photodiode is not

capacitance. Use optics to concentrate light on a

exposed to any light, and respond without the added

small photodiode.

delay that results from coming out of the negative rail;

3. Noise increases with increased bandwidth. Limit

this configuration is shown in Figure 41. This bias

the circuit bandwidth to only that required. Use a

voltage

also

appears

across

the

photodiode,

capacitor across the RF to limit bandwidth, even if

providing a reverse bias for faster operation.

not required for stability.

4. Circuit

board

leakage

can

degrade

the

performance

otherwise

well-designed

amplifier. Clean the circuit board carefully. A

circuit board guard trace that encircles the

summing junction and is driven at the same

voltage can help control leakage.

For additional information, refer to the Application

Bulletins Noise Analysis of FET Transimpedance

Amplifiers (SBOA060), and Noise Analysis for High-

Speed Op Amps (SBOA066), available for download

at the TI web site.

HIGH-IMPEDANCE SENSOR INTERFACE

Many sensors have high source impedances that

may range up to 10M

Ω, or even higher. The output

signal

sensors

often

must

amplified

(1) CF is optional to prevent gain peaking. It includes the stray

otherwise conditioned by means of an amplifier. The

capacitance of RF.

input bias current of this amplifier can load the sensor

output and cause a voltage drop across the source

Figure 41. Single-Supply Transimpedance

resistance, as shown in Figure 42, where (VIN+ = VS –

Amplifier

IBIAS × RS). The last term, IBIAS × RS, shows the

voltage drop across RS. To prevent errors introduced

For additional information, refer to Application Bulletin

to the system as a result of this voltage, an op amp

(SBOA055), Compensate Transimpedance Amplifiers

with very low input bias current must be used with

Intuitively, available for download at www.ti.com.

high impedance sensors. This low current keeps the

error contribution by IBIAS × RS less than the input

OPTIMIZING THE TRANSIMPEDANCE

voltage noise of the amplifier, so that it does not

CIRCUIT

become the dominant noise factor. The OPA320

series of op amps feature very low input bias current

To achieve the best performance, components should

(typically 200fA), and are therefore ideal choices for

be selected according to the following guidelines:

such applications.

1. For lowest noise, select RF to create the total

required gain. Using a lower value for RF and

adding gain after the transimpedance amplifier

generally produces poorer noise performance.

The noise produced by RF increases with the

square-root of RF, whereas the signal increases

linearly. Therefore, signal-to-noise ratio improves

when all the required gain is placed in the

transimpedance stage.

2. Minimize

photodiode

capacitance

and

stray

capacitance at the summing junction (inverting

Figure 42. Noise as a Result of IBIAS

input). This capacitance causes the voltage noise

of the op amp to be amplified (increasing

amplification at high frequency). Using a low-

noise

voltage

source

reverse-bias

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